1. Field of the Invention
This invention relates to superlattice semiconductor devices, and more particularly to antimony-based superlattices that are doped n-type with silicon or tin, and to related fabrication methods.
2. Description of the Related Art
Antimony (Sb)-based compounds have shown considerable potential for far-infrared detector applications and as heterojunction materials that can be used with InAs channel high electron mobility transistors (HEMTs). Certain antimony-based compounds have a small bandgap energy and a corresponding high electron mobility, making them useful for high speed devices. Group III-Sb compounds, particularly GaSb and AlGaSb are of particular interest.
Silicon is the most commonly used n-type dopant for Group III-V molecular beam epitaxy (MBE) growth processes; tin is another suitable n-type dopant. Silicon's low vapor pressure makes it easily controllable in the normal flux source temperature range used in standard MBE processing. However, silicon is not amphoteric in antimony compounds, but acts as a p-type dopant, and cannot be used to produce n-type material. On the other hand, Group VI impurities such as S, Te and Se are well-behaved n-type impurities for GaSb and other III-Sb compounds. "Captive source" configurations are currently used in which the Group VI impurity is evaporated from a lower vapor pressure compound, such as Te from a PbTe source. The use of Group VI n-type dopants is discussed in Sunder et al., "Czochralski Growth and Characterization of GaSb", Journal of Crystal Growth, Vol. 78, No. 9, 1986, pages 9-18. Unfortunately, Group VI materials have very high vapor pressures, and accordingly have a strong tendency to leave residual amounts of the Group VI material in the MBE apparatus. This residue contaminates the MBE machine and interferes with later p-type processing with the same machine.
In the silicon or Group VI doping mentioned above, the dopant impurity is introduced directly into the material to be doped by opening shutters for both the dopant and for the material being doped during the MBE growth process. A modified doping technique that has been used for HEMTs is referred to as modulation doping, and is described in Dingle et al., "Electron mobilities in modulation-doped semiconductor heterojunction superlattice", Applied Physics Letters, Vol. 33, No. 7, 1 Oct. 1978, pages 665-667. The typical application of modulation doping is in the n-type doping of GaAs. It would be desirable to dope the GaAs with silicon, but silicon doping produces impurity scattering in the GaAs. Instead of directly doping it with silicon, alternating layers of GaAs and a generally lattice matched material, typically AlGaAs, are grown. The AlGaAs layers, but not the GaAs layers, are doped with silicon. This produces a concentration of mobile electrons in the AlGaAs that, because of the higher electron affinities of the adjacent GaAs layers, drift into those layers. The drift of electrons into the GaAs layers avoids the impurity scattering associated with a direct doping of the GaAs with silicon, and thereby increases the device's mobility and speed. However, this type of doping is not directly applicable to Group III-Sb compounds, since such compounds are not closely lattice matched with either GaAs or AlGaAs.
Modulation doping of InAs by impurity-doped AlGaSb layers has also been reported in Tuttle et al., "Effects of interface layer sequencing on the transport properties of InAs/AlSb: Evidence for antisite donors at the InAs/AlSb interface", Journal of Applied Physics, Vol. 67, No. 6, 15 Mar. 1990, pages 3032-3037. Thus far, however, the reported doping and high mobilities have not been found to be reproducible. Furthermore, electron mobility dropped more than usual as the dopant concentration increased, apparently because of defects near the interface, thus limiting the potential operating speed.
One type of HEMT that combines lattice-mismatched materials is discussed in Wang ed, Introduction to Semiconductor Technology-GaAs and Related Compounds, Chap. 2 by Shor, Chap. 3 by Pei et al., John Wiley & Sons, 1990, pages 67-71, and 148-152. It involves an AlGaAs/InGaAs/GaAs structure with an InGaAs channel layer that is kept fairly thin, on the order of 150 Angstroms. The advantage of such structures is the higher electron velocity and mobility of InGaAs. The rather large lattice mismatches are accommodated by elastic deformations in the thin heterostructure layers, with a lattice-mismatched InGaAs channel grown on either GaAs or InP substrates; AlSb/InAs quantum well HFETs have also been realized. The relatively thin channel layer that has been elastically deformed is referred to as a strained layer. However, this type of strained layer device does not solve the problem of n-type doping for antimony based compounds used for the source and drain layers.